Tetrafunctional porphyrins-containing trithiocarbonate groups were synthesized by an ordinary esterification method. This tetrafunctional porphyrin (TPP-CTA) could be used as a chain transfer agent in a controlled reversible addition-fragmentation chain transfer (RAFT) radical polymerization to prepare well-defined 4arm star-shaped polymers. N,N-Diethylacrylamide was polymerized using TPP-CTA in 1,4-dioxane. Poly(N,N-diethylacrylamide) (PDEA) is known to be a thermo-responsive polymer, and exhibits a lower critical solution temperature (LCST) in water. The star-shaped PDEA polymer (TPP-PDEA) was therefore also thermo-responsive, as expected. The LCST of this polymer depended on its concentration in water, as confirmed by turbidity, dynamic light scattering (DLS), static light scattering (SLS), and 1 H NMR measurements. The porphyrin cores were compartmentalized in PDEA shells in aqueous media. Below the LCST, the fluorescence intensity of TPP-PDEA was about six times larger than that of a water-soluble low molecular weight porphyrin compound (TSPP), whose fluorescence intensity was independent of temperature. Above the LCST, the fluorescence intensity of TPP-PDEA decreased, while the intensity was about three times higher than that of TSPP. These observations suggested that interpolymer aggregation occurred due to the hydrophobic interactions of the dehydrated PDEA arm chains above the LCST, with self-quenching of the porphyrin moieties arising from these interactions.